When the sort order of a image gallery is set to "priority" and some images are set to the same priority, in the full view of those images the navigation ("previous" or "next" links) is not working.

Steps to reproduce

in 3.9.1 with webin fresh installed:
1) in the admin interface go to the "cars" image gallery;
2) change the gallery sort order to "priority" and leave all priorities to 0;
3) clear cache;
4) in the public site go to the "cars" image gallery in the "community" section and view a full image;
5) the navigation links ("previous image", "next image" links) are not linked, ever if the gallery contains 4 images
in 3.8.7 you must configure a siteacces with the "base" design and create an object of class "gallery" with some images with in, then follow similar above steps.
The bug is present when some images have the same priority value.